Composite Materials Research Progress

244
Maria Pia Cavatorta and Davide Salvatore Paolino
events [40]. Strictly speaking, the DD is not a damage variable but rather a point by point
measurement of the laminate efficiency of energy absorption, whereas the coefficient η
proposed by Liu averages the efficiency of energy absorption over a wide range of impact
energies (from very low energies to Pn).
E a/P n
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
GVP90_12.31
GE90s_8.00
GE90m_8.00
GE45_4.50
GE90s_4.00
GE90m_4.00
CE60_1.75
CE90_1.55
CE60_0.85
CE90_0.75
CE60_0.40
CE90_0.35
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
E i/P n
Figure 3. Energy absorption Master Curve.
Figure 3 plots the Master Curve proposed in [19]. Data for the three thicker laminates are
again superimposed and very close to the bisector, meaning that the absorbed energy is about
equal the impact energy (DD almost one). For thin laminates, the difference is more enhanced
pointing out a lower efficiency of energy absorption [3].
As said, no indications can be evinced from Figures 2 and 3 in terms of laminate damage
tolerance. In this respect, definition of the DuI, which differentiates between the nature of
energy absorption mechanisms, appears rather appealing. In its original definition, the DuI is
meant to rank different laminates tested under similar impact conditions, on the basis of a
more fragile or more ductile behavior under impact loading. It is worthwhile noticing that in
the literature the DuI is basically used to rank different laminates at perforation or fracture
(Charpy tests), for which determination of Einitiation and Epropagation poses no trouble. The forcedisplacement
curves are open curves and Einitiation is calculated without ambiguity as the area
bounded by the force-displacement curve up to Fpeak and the horizontal axis, while Epropagation
is calculated as the area bounded by the force-displacement curve from Fpeak to a constant
level of force and the horizontal axis (Figure 4a). In the attempt of proving the DuI against
other damage variables, computation of the DuI was also applied to impact tests with
rebound. When the dart rebounds, the force-displacement curves are closed curves and
computation of Einitiation and Epropagation becomes troublesome as different areas could be
considered. No references of DuI computation in impact tests with rebound were found in the
literature. Considering that in impact events with rebound the energy absorbed by the
laminate is equal to the area bounded by the force-displacement curve, in computing the DuI